EARTH - The Third Planet from the Sun. Sounds like the name of a sit-com. Oh wait, it is. Now you know where the name came from. Seriously -- I do find it interesting that in many languages, including English, the world for our planet has been the word for -- well Earth -- dirt, ground, "the firmament".

The bigger question about our planet, is "

This beautiful ball in the sky, why and how did life form here, and not on the other inner planets, Mars or Venus? I know that we cannot rule out primitive life (microbial for example), on those planets -- but we teem.

We have oceans,

an atmosphere, and weather patterns conducive to life just about everywhere -- from the deepest depths of the ocean to the coldest regions to the hottest regions, life is all around us.

Earth teems with life.

Incredible! but how and why?

Panspermia and exogenesis are theories proposing that life originated elsewhere in the universe and was subsequently transferred to Earth in the form of spores perhaps via meteorites, comets or cosmic dust. However those theories do not help explain the ultimate origin of life.

As a planet, we are about the same size as Venus - about 9% larger. We are 22m miles further from the sun -- not that much. We are 50m miles closer to the sun than Mars, And both are very different from us. Venus is pressured by intense greenhouse gases, so while you would weigh about the same on Venus as on Earth, you would only weigh that for a second or two, till you were crushed as flat as a pancake. A heavy pancake but...

On Mars, the atmosphere is thin, and while we are discovering organic processes (Water, Ice, Methane, for example), we do not understand what has happened on this planet to essentially, wipe away the atmosphere.

So there is the mystery that with all the great minds of the world over time, we have not got a definitive answer.

Inner Planets

There are many theories: We were seeded by comets that carried water, God created us during some time off, life just developed here.

For you younger -- or older -- students, there is an
avocation waiting for you. Figure this out.

Some details about us. Earth is just under 24,000 miles in diameter. Ring a bell. It's why we fixed clocks to have 24 hour days. (almost -- there are now occasional leap seconds). This too, was not always the case: The interaction of the Earth and the Moon slows the Earth's rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 18-hour days in a year.

We are almost a perfect sphere, so we divide up longitudes by 15 degrees (360 / 24). This means every time zone - one hour roughly - is 15º around the planet. We have divided north and south hemispheres into 10 degree latitudes. The equator is 0º. The Tropic of Cancer is (today) set at 23.5º north and the Tropic of Capricorn -- 23.5º south. As our climate continues to change, these tropical latitudes will have to be revisited.

Now, before we take our comfort zone for granted, here's some information that should make you ponder, and hopefully, turn you into a scientist or engineer, and not a lawyer.


While we have an

The atmosphere keep air inside so we can breathe, and keeps out much of the Solar Radiation, that could cause us harm. The two functions of the atmosphere are a delicate balance.

The Atmosphere is not very thick and it is most fragile. The height of the atmosphere (up to the Mesosphere) is about the distance from London to Birmingham, New York to Philadelphia, Sydney to -- well nowhere. It's about 50 miles high. Of that 50 miles, the 25 miles closest to the ground contain 85% of the atmospheric elements that protect us from solar radiation, and essentially provide us life. The well-known "Ozone Layer" --that shields us from harmful solar radiation is only 35 miles high.

Naturally, this narrow band is vulnerable to any kind of change, temperature, chemical components, being penetrated by asteroids and/or comets.

So, our atmosphere requires care and attention -- constantly. Here is what the atmosphere looks like from the Space Station:

Looking Down at Earth

Next, Earth itself is an organic object. Even if it wasn't full of flora and fauna, it would be changing constantly over the millennia. There have been periods where much of the planet was extensively covered in
ice (ages), and periods where the tropics extended much farther north and south than they do now.

The Antarctic

The picture above is a panorama of an ice field in the Antarctic. Below is a photograph of Scandinavia that shows the effect of ice ages:


Although the last glacial period ended more than 8,000 years ago, its effects can still be felt today. For example, the moving ice carved out landscape in Canada, Greenland, northern Eurasia and Antarctica. The erratic boulders, till, drumlins, eskers, fjords, kettle lakes, moraines, cirques, horns, etc., are typical features left behind by the glaciers.

There is more about this in the Earth section of the site, but remember, we humans have been migrating for more than 70,000 years and the last ice age -- with its land bridges only ended 8000 years ago. This helps explain how our ancestors moved.


While the atmosphere of Earth is fragile and needs care and repair, the interior of Earth is violent and constantly shifting. Deep inside the Earth is molten rock (the Mantle), that is heated by a deeper core of molten iron. Earth's top crust is divided into several separate solid plates which float around independently on top of the hot mantle below. When fractures occur, there are fissures that form allowing the "lava" to bubble to the surface. These can be both above ground and below the ocean volcanoes.

The image below shows what we think is the present interior of the Earth, combined with our thin atmosphere above us:

Earth Crust Cutaway

The Earth is divided into several layers which have distinct chemical and seismic properties (
depths in km):

0 - 40 Crust
40- 400 Upper Mantle
400- 650 Transition region
650-2700 Lower mantle
2700-2890 D'' layer
2890-5150 Outer core
5150-6378 Inner core

The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.
Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms):

atmosphere = 0.0000051
oceans = 0.0014
crust = 0.026
mantle = 4.043
outer core = 1.835
inner core = 0.09675

The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by mass) is:

This site offers a complete interactive list of the chemical elements that have been found on Earth. The proportions are as follows:

34.6% Iron

As a result of the chemical composition of Earth, it is the densest major body in the solar system. - despite the relatively small scale when compared to the giant planets (Jupiter, Saturn, Uranus and Neptune).

At the surface of our planet, there are giant (tectonic) plates of rock that "float" on the mantle, and move around.

These plates overlap and shift over LONG times. When they push together, and upwards, we get mountain ranges, when they shift -- horizontally or vertically, we get "Earthquakes". Under sea earthquakes can create enormous waves in the oceans resulting in what we have come to call Tsunami -- like a huge ripple that you get when you fold a sheet.The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia).

Earth's Current Tectonic Plates

As you can see from the map above, there are (at present) eight major plates:

North American Plate - North America, western North Atlantic and Greenland Earth's Plate Boundaries delineated by earthquake epicenters
South American Plate - South America and western South Atlantic
Antarctic Plate - Antarctica and the "Southern Ocean"
Eurasian Plate - eastern North Atlantic, Europe and Asia except for India
African Plate - Africa, eastern South Atlantic and western Indian Ocean
Indian-Australian Plate - India, Australia, New Zealand and most of Indian Ocean
Nazca Plate - eastern Pacific Ocean adjacent to South America
Pacific Plate - most of the Pacific Ocean (and the southern coast of California!)

There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundaries.

The Earth's surface is very young. In the relatively short (by astronomical standards) period of 700,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters).

Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare. The oldest fossils of living organisms are less than 3.9 billion years old. There is no record of the critical period when life was first getting started.

A brief history of Tectonic Plates
If you click on the image to the left, you should see a short video that shows what we think has happened to the Earth's Tectonic plates over the last 700 million years. Remember that humans showed up about 3.5 million years ago, and our branch (couch potatoes) about 100,000 years ago.

If you stopped that animation at about 200 million years ago, you would see that our present continents fitted together and we call this uni-continent Pangaea.

This is what we think it looked like back then:


All in all, Earth is a magnificent creation, but it is a young planet, prone to change, both in structure and in resulting climate.

With a burgeoning population we are presented with additional challenges -- how to sustain humanity -- without upsetting the delicate Earth eco-system. We are desperate for good minds to study the planet, to build early warning systems of change, and to help leaders and people make necessary adaptations.

We have life here now, but we should not take it for granted.


Fomal Name: Terra, Sol III - Earth is the third planet from the Sun and the fifth largest:

orbit: 149,600,000 km (approximately 93,000,000 miles (1.00 AU) from Sun
diameter: 12,756.3 km
mass: 5.972e24 kg

It was not until the time of Copernicus and Galileo (the sixteenth century) that it was understood that the Earth is just another planet and not a flat land.

This is an image taken from a Space Shuttle on it's way to
Mir, the Russian Space station of the 1990s.

Earth and Mir

Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful.

The other terrestrial planets probably have similar structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.

Space Shuttle view of the Strait of Gibraltar (an example of a land split)

the Strait of Gibraltar

71 Percent of the Earth's surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface (though there may be liquid ethane or methane on Titan's surface and liquid water beneath the surfaces of Mars and Europa). Liquid water is, of course, essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable. Liquid water is also responsible for most of the erosion and weathering of the Earth's continents, a process unique in the solar system today (though it may have occurred on Mars in the past).

Earth's atmosphere seen at the limb:

Earth's atmosphere seen at the limb

And as seen with a shuttle approaching Mir (you can see the narrow band of our atmosphere clearly in this photograph:

Mir from Shuttle Missiom STS-63

The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. There was probably a very much larger amount of carbon dioxide in the Earth's atmosphere when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants. Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to these various "sinks" and back again.

The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth's surface temperature via the greenhouse effect. The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid -21 C to a comfortable +14 C); without it the oceans would freeze and life as we know it would be impossible. (Water vapor is also an important greenhouse gas.)

The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under "normal" circumstances would quickly combine with other elements. The oxygen in Earth's atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen.

Earth has a modest
magnetic field produced by electric currents in the outer core. The interaction of the solar wind, the Earth's magnetic field and the Earth's upper atmosphere causes the auroras. Irregularities in these factors cause the magnetic poles to move and even reverse relative to the surface; the "geomagnetic north pole" is currently located in northern Canada.

Southern Aurora

The Earth's magnetic field and its interaction with the solar wind also produce the
Van Allen radiation belts (see image below), a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth. The outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude.

The Van Allen Radiation Belt

These belts help deflect solar radiation, allowing us to be couch potatoes and not potato chips.